### Author Topic: LEC5101 Question  (Read 508 times)

#### yingxuanwang

• Newbie
• Posts: 3
• Karma: 0
##### LEC5101 Question
« on: September 19, 2020, 12:46:43 AM »
Hi, I have a question when I solve the first order equation.
I have found that sometimes to multiple a factor to the equation to construct a exact equation is much more convenient to use formula to solve it. But it is hard for me to find such a integral factor, is there any techniques to use this method? Or how can I know when I should use such method?
Thanks.

#### Victor Ivrii

• Elder Member
• Posts: 2570
• Karma: 0
##### Re: LEC5101 Question
« Reply #1 on: September 19, 2020, 07:21:13 AM »
You need to wait lectures ; there will be shown some recepies  to find integrating factors, but there is no general algorithm

#### RunboZhang

• Sr. Member
• Posts: 51
• Karma: 0
##### Re: LEC5101 Question
« Reply #2 on: September 19, 2020, 09:59:38 AM »
I think a general idea behind integrating factor is the use of product rule. Since a general form is u(t)dy/dx +u(t)p(t)y = u(t)g(t), and we can observe a pattern of u'y+uy' on LHS. Thus the LHS becomes d(u(t)y)/dt, and it makes the equation easier to solve. More details are expanded in the textbook starting from P24 and those illustrations and examples are quite useful. I don't know if I have addressed your problem, if I did not make my point clear please comment below.